The objective of this project is to examine the mechanism of transport and factors that influence of organic anions across dog and rat renal cortical membranes. Brush-border (luminal) and basal-lateral (contraluminal) membranes will be isolated by differential centrifugation and separated by free-flow electrophoresis. Initial experiments utilizing radiolabeled citrate demonstrated sodium-citrate cotransport in luminal, but not contraluminal membranes of dog kidney. Contraluminal transport, however, was stimulated by inwardly directed sodium gradients. In both membranes, citrate transport was saturable and exhibited accelerated homeo and heteroexchange diffusion. These collective data indicate that citrate transport is carrier-mediated in both luminal and contraluminal membranes of dog renal cortex. The proposed studies will evaluate the question of species differences in citrate transport by assessing transport in membrane vesicles of rat kidney. In view of the finding of heteroexchange diffusion between citrate and PAH, the proposed studies will also address the possibility that a common organic anion transporter is present in these membranes. Accordingly, the interaction of citrate and other Kreb's cycle organic anions will be examined. Since PAH and urate may share a common carrier, the relationship between citrate and urate, and inhibitors of urate transport (pyrazinoate, oxonate) will also be studied.